1. Field of the Invention
The present invention relates to an electro-mechanical-acoustic transducer for generating a ringing tone or a ringing vibration for a portable terminal apparatus such as a portable telephone, a pager or a PHS (personal handy phone set), and a method for driving the same.
2. Description of the Related Art
Conventionally, a portable terminal apparatus such as a portable telephone, a pager or a PHS includes a plurality of vibrators which are connected to an electric signal generator so as to selectively generate a bell tone or a vibration for alerting the user of an incoming call.
There is a demand in the art to reduce the size and weight of a portable terminal apparatus. For example, Japanese Laid-Open Publication No. 8-275293 of Motorola, Inc., discloses a portable telephone using a single unit electro-mechanical-acoustic transducer capable of generating a tone and a vibration.
FIG. 32 illustrates a structure of such a conventional electro-mechanical-acoustic transducer 3200. Referring to FIG. 32, a movable mass 1 is supported by a planar nonlinear spring member 2. An electromagnetic coil 4 is enclosed in a coil form 3. A permanent magnet 5 is fixed to the periphery of the movable mass 1 opposite to the electromagnetic coil 4. Although not shown in FIG. 32, the periphery of the coil form 3 is attached to a soundboard (e.g., a casing of a portable telephone).
In the electro-mechanical-acoustic transducer 3200, an alternating magnetic field is generated in response to a drive signal applied to the electromagnetic coil 4, thereby producing an alternating excitation force between the electromagnetic coil 4 and the permanent magnet 5 thus vibrating the movable mass 1. This vibration is transmitted to the soundboard, thereby providing a vibratory alert signal when the portable telephone receives an incoming call. The transducer 3200 also generates an audible signal based on a similar principle. In order to obtain a large vibration force, a planar nonlinear spring member is employed as the member 2 for supporting the movable mass 1. FIG. 33 is a graph illustrating the relationship between the displacement and the frequency of a planar nonlinear spring member. It is known in the art that a nonlinear spring member exhibits a vibration characteristic called a “jumping phenomenon” (see, for example, YOSHIHISA “Speaker,” Rikokenkyusha (March 1973). This is a phenomenon in which the displacement of the nonlinear spring member changes from A→B→C→D when the frequency is increased, and from D→C→E→F→A when the frequency is decreased.
In the F–B frequency range, the displacement characteristics are influenced by the direction of the applied frequency signal (i.e., whether it is increased or decreased). Thus, a stable vibration cannot be obtained. In view of this, the conventional electro-mechanical-acoustic transducer 3200 uses a drive signal which sweeps in the A–F frequency range, where the displacement characteristics are not influenced by the direction of the frequency signal. Where the A–F frequency range is employed, the vibration force is large at frequency F and very small at frequency A. Therefore, the vibration output obtained while the frequency signal sweeps across the A–F frequency range may not be efficient. Particularly, for a mobile information terminal apparatus such as a portable telephone, whose driving source is a battery, a strong desire exists to maximize the efficiency of a transducer and thereby reduce the power consumption thereof, so as to allow the apparatus to be used for a long period of time.